Molybdenum oxide refining process

ABSTRACT

A process for effecting the economical removal of various impurities from roasted molybdenite concentrates by extracting the molybdenum trioxide content in an aqueous ammonium hydroxide solution and effecting further conversion and extraction of the molybdenum value in the insoluble solid residue, thereby producing a molybdenum trioxide product of greater than 99 percent purity and effecting a recovery of substantially all of the molybdenum values in the original crude material.

United States Patent 2 Barry et a1.

1451; Apr. 25, 1972 [54] MOLYBDENUM OXIDE REFINING PROCESS 1 [72] Inventors: Henry F. Barry; Ctilvln Ballade; Jerry D. Baker, allot Ann Arbor, Mich,

American Metal Climax, Inc., New York, NY.

221 Filed: May 15,1970.

211 Appl.No.: 37,674

[73] Assignee:

[52] U.S.Cl ..23/1 5 W,23/51 R, 23/140,

Kunda .t ..23/l5 W lredell.....

Donahue et a1. ..23/15 W Primary Examiner-HerbertT. Carter Attorney-Harness, Dickey 81 Pierce 57 ABSTRACT.

A process for effecting the economical removal of various impurities from roasted molybdenite concentrates by extracting the molybdenum trioxide content in an aqueous ammonium hydroxide solution and effecting further conversion and extraction of the molybdenum value in the insoluble solid residue, thereby producing a molybdenum trioxide product of greater than 99 percent purity and effecting a recovery of substantially all of the molybdenum values in the original crude material.

7 Claims, 2 Dravvlng Figures Patented April 25, 1972 2 Sheets-Sheet a Mm ZNF m F w md 0 53? 62% Z7 7 m3 5% I Patented April 25, 1972 2 Shoots-Shut 2 llllv 1- MOLYBDENUM OXIDE REFINING PROCESS BACKGROUND or THE INVENTION Molybdenum is principally found in the earths crust in the 9 form of molybdenite (M08,) as soft, hexagonal, black flaky crystals. The molybdenite is conventionally distributed'in the form of very fine veinlets in quartz present in an ore body comprised predominantly of an. altered and highly silicified granite. Generally, the concentration of molybdenite in such ores, as mined, is in the order of about 0.5 percent. The

' molybdenite can be extracted by anyone of a variety of known beneficiation processes to increase its concentration to a level generally upwards of about 80 percent by weight. Typical of such known ore beneficiation processes is oil flotationextraction operations in which the ore is preliminarily ground poses including lubricants, the predominant portion of such oreconcentrates are converted to the oxide form in which they are utilized as alloying constituents in various metallurgical processes or can be further reduced to the pure metallic state for special uses. A conversion of the ore concentrate to the oxide form is conventionally achieved by subjecting the concentrate to a roasting operation in which the ore is placed on a circular open hearth-type furnace, such as a Herreshoff furnace, in which the concentrate is heated in the presence of excess air, forming the molybdenum trioxide product, and wherein the sulfur dioxide is released as a gaseous by-product. During the course of the roasting operation, the concentrate is moved in a cascading type fashion from the. uppermost to lowermost hearth of the multiple-hearth furnace from which it is removed as a roasted concentrate generally containing upwards of about 85 percent molybdenum trioxide.

Ordinarily, such roasted concentrates are satisfactory for use without further refining or treatment. In some instances, however, a further refinement of the molybdenum trioxide roasted concentrate is desirable to remove the various impurities which vary in concentration depending on the particular source of the molybdenite ore. Typical of the impurities present in such roasted concentrates are lead, iron, zinc, bismuth, calcium, aluminum, potassium and silica, of which lead constitutes one of the more objectionable constituents. Processes heretofore used or proposed for use in effecting the removal of predominant proportions of such contaminating constituents, thereby providing an upgraded molybdenum trioxide product, have been unsatisfactory in one or more respects and, in particular, have been commercially uneconomical to perform due to the costs attending the purification process and the comparatively high loss of molybdenum trioxide originally present in the roasted concentrate.

In accordance with the present process, it is now commercially feasible to effect a removal of the predominant proportion of impurities present in roasted molybdenum oxide concentrates, providing an improved technical oxide product comprising usually at least about 99 percent molybdenum trioxide and having a residual lead content usually less than about 20 parts per million (ppm). Moreover, the process comprising the present invention provides for a recovery of 99.5 percent or more of the original molybdenum in the feed, which substantially enhances the economy of the purification process.

SUMMARY OF THE INVENTION The benefits and advantages of the present invention are achieved by a process in which a purification of conventional roasted concentrates is achieved by contacting such concentrates with an aqueous ammonium hydroxide solution for 'a to a relatively fine particle size, usually less than about 200 2 period of time sufficient to extract the predominant portion of the molybdenum trioxide constituent thereof by converting it to soluble ammonium molybdate and extracting the undissolved solid residuetherefrom, which is subjected to a further oxidation treatment to effect conversion of substantially all of the remaining molybdenum'to the trioxide form. The resultant oxidized residue is also treated withan aqueous ammonium hydroxide solution to effect an extraction of the molybdenum trioxide as ammonium molybdate, which subsequently is recovered by recrystallization from the ammonium hydroxide solution. The resultant recovered, crystals are subjected to a calcining step in which they are heated to an elevated temperature for a period of time sufficient to effect a decomposition of the crystals and a liberation of ammonia and water, producing a molybdenum trioxide product which is conventionally composed of at least about 99.0 percent molybdenum trioxide and contains less than about 20 ppm lead. The solid residue from the oxidation step is subjected to a scavenging treatment to recover the residual molybdenum value therein as a calcium molybdate product.

Additional benefits and advantages of the present invention will become apparent upon a reading of the description of the preferred embodiments taken in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 comprises a schematic flow diagram of the process for effecting a purification of the roasted concentrate in accordance with the preferred embodiments of the present in- DESCRIPTION OF THE PREFERRED EMBODIMENTS The compositions of the various materials described in connection with practice of the process comprising the present invention are herein defined in terms of percentages by weight, unless otherwise specified.

Referring now in detailto the drawings, the process for effecting a purificationof the molybdenum oxide roasted concentrate, as schematically shown in FIG. 1, comprises three major sections designated as preparation section, product recovery section and scavenging section, respectively. In the preparation section,and extraction of up to about 99 percent of the molybdenum value in the roasted concentrate is achieved by effecting a conversion thereof into a soluble ammonium molybdate. In the product recovery section, the ammonium molybdate is separated from the aqueous ammoniacal solution in the form of crystals which are subsequently calcined so as to yield a high purity molybdenum trioxide product. Finally, in the scavenging section, a treatment of the residual solids derived from the preparation section is performed to extract the predominant portion of the residual molybdenum values therefrom, producing a calcium molybdate by-product. The combination of the three process sections provides for a recovery of upwards of 99.5 percent of the molybdenum values originally present in the roasted concentrate feed.

The preparation section, as shown in FIG. 1, includes a tank or dissolver indicated at 2, into which the roasted concentrate and anhydrous ammonia, as a make-up, is charged, forming an ammoniacal solution. The dissolver 2 is equipped with suitable agitation for maintaining the particles in suspension to assure good liquid/solid contact. The roasted concentrate can vary in composition depending on the particular source of the molybdenite ore and the particular type and degree of beneficiation processing to which the ore has been subjected. Typically, the roasted concentrate comprising the feed material to the dissolver will contain about 87 percent molybdenum trioxide, 4.5 percent molybdenum dioxide, about 8 percent of insoluble contaminants such as silica, alumina, calcium, lead, iron, and

zinc, and about 0.5 percent soluble contaminants such as copper, potassium and sodium. The concentrate itself is in the form of a finely particulated solid, which conventionally will pass through a 50 mesh screen, although such concentrates can contain agglomerates which have become fused during the roasting process which may range up to about one-half inch in diameter. Generally, it is preferred to further crush such agglomerates to reduce their size, thereby exposing increased surface area for reaction with the aqueous ammonium hydroxide solution.

The roasted concentrate feed can be fed to the dissolver 2 in a continuous or batchwise manner so as to effect a conversion of the molybdenum trioxide constituent to ammonium molybdate in accordance with the following general equation:

The ammonium molybdate formed is extremely soluble in the aqueous ammonium hydroxide solution.

The aqueous solution contained in the dissolver 2 incorporates a concentration of ammonia sufficient to effect a conversion of substantially all of the molybdenum trioxide to the corresponding ammonium molybdate soluble compound. Particularly satisfactory results are obtained when the molar ratio of the ammonia-to-molybdenum is maintained within a range of about 2:1 to about 2.5: l and preferably at about 2.3: l. The aqueous ammonia solution is prepared from the ammonium hydroxide filtrate recycled from the filter 12 back to the dissolver 2, from the make-up anhydrous ammonia added to the dissolver, from ammonia solution recovered as condensate from the crystallizer l6 and condenser 20, in addition to water make-up, so as to preferably maintain the ammonia content of the solution at a concentration of about 12.5 percent by weight, providing a molar ratio of ammonia-to-molybdenum of 2.3: l for a slurry almost saturated with molybdenum.

The concentration of the roasted molybdenite concentrate in the initial feed can range from 25 to 40 percent with a preferred solid-to-liquid feed ratio of 2:1. Employing a solution containing 12.5 percent ammonia under these conditions will conventionally result in a dissolver solids content at equilibrium in the dissolver unit of about percent. The concentration of insoluble solids in the dissolver unit is preferably controlled within a range of 1 percent or 2 percent up to about 20 percent by weight, based on the total slurry. Preferably, during a continuous processing arrangement, an average slurry insoluble solids content of about 5 percent is maintained in the dissolver, thereby achieving optimum utilization of equipment, while concurrently assuring a good liquid-to-solid surface contact. The dissolver, when operated on a continuous basis, is sized relative to the volume of slurry passing therethrough so as to provide an average residence time of about minutes to assure a substantially complete reaction and solution of the molybdenum trioxide constituent in the roasted concentrate. The contact time can be varied between as little as 5 minutes to as long as 60 minutes, although contact periods in excess of about minutes are usually not economical. As indicated by the foregoing reaction equation, the reaction and dissolution of the molybdenum trioxide as ammonium molybdate is exothermic, requiring temperature control of the solution. The temperature of the aqueous solution is not critical provided that the temperature is not to high, tending to cause volatilization of the ammonia constituent. While temperatures ranging from 25 to about l00 C. can be used, particularly satisfactory results have been attained when the temperature of the ammoniacal solution is controlled within a range of from about 50 to about 75 C., and preferably 60 C.

All or a portion of the slurry containing the residual solids and the aqueous solution containing the dissolved ammonium molybdate is transferred on a batchwide or continuous basis from the bottom of the dissolver to a thickening or settling tank 4 for effecting a coarse separation between the residual solids and aqueous solution. Alternatively, a portion or all of the slurry from the dissolver can be directly transferred to a filter 6, thereby circumventing the settling tank 4. The undertlow from the settling tank 4 is transferred in a manner as shown in FIG. 1 to a filter 6, such as a rotary drum-type filter. The filtrate from the filter 6 containing the dissolved ammonium molybdate is combined with the supernatant liquid withdrawn from the upper portion of the settling tank 4 and the resultant combined stream is transferred to a filter 14 of the product recovery section. The filter cake of the filter 6 can be washed, if desired, and the wash solution returned to the dissolver in combination with the filtrate stream from the filter unit 12. The washing of the filter cake in the filter 6 constitutes an optional step and is effective to remove the residual soluble ammonium molybdate entrapped therein, which usually comprises about one-half of the total molybdenum content remaining in the residual solids.

Employing typical roasted concentrates which are treated in the dissolver in accordance with the preferred conditions as hereinbefore set forth has resulted in the formation of filtrates as transferred to the filter 14 which are substantially saturated with molybdenum and contain from about 250 to 270 grams per liter molybdenum. When iron constitutes an appreciable contaminant of the roasted concentrate, such filtrates have a characteristic rust color which, upon settling or after further filtration, results in a recovery of iron solids generally ranging from about 0.6 to 1.2 grams per liter. The resultant clarified solution containing the dissolved ammonium molybdate usually ranges from a light to a dark blue color which is dependent on the quantity of a copper-amine complex ion present in the solution. When copper and zinc are present in appreciable quantities and/or their presence is not desired, a removal of such copper and zinc complexes from the filtrate can be achieved by employing sulfides, such as ammonium sulfide or hydrogen sulfide, effecting a reaction with and a precipitation of the corresponding copper and zinc sulfides from the solution.

Analyses conducted on five runs employing typical roasted concentrates reveal that approximately percent of the molybdenum originally present in the concentrate is converted to ammonium molybdate as a soluble constituent in the filtrate. Analyses of such runs reveal the following percentages of molybdenum removed from the concentrate: 87.4%, 88.3%, 91.4%, 91.1% and 93.6%. Still further improvement in the recovery of molybdenum from the original roasted concentrate is achieved by subjecting the filter cake collected in the filter 6 to wash step whereby increases ranging from 2.6 up to 6.1 percent in the total amount of molybdenum extracted where obtained.

The filter cake recovered from the filter 6 conventionally contains about 30 percent water and around 2 to 4 percent residual ammonia. The predominant proportion of the molybdenum content in the filter cake is in the form of molybdenum dioxide, which must be further oxidized to molybdenum trioxide in order to be extracted by additional aqueous ammonium hydroxide solution. For this purpose, the filter cake can be dried and subjected to further roasting in the presence of air to effect the requisite oxidation reaction or, alternatively, the conversion to molybdenum trioxide can be achieved by a liquid phase oxidation employing an autoclave 10, as shown in the preparation section of FIG. 1. Still another alternative is to repulp the filter cake and extract the molybdenum dioxide particles in accordance with the process schematically illustrated in FIG. 2 and as subsequently described herein.

When a liquid phase oxidation reaction is to be performed on the filter cake in accordance with the arrangement shown in FIG. 1, the solids are transferred from the filter 6 to a tank 8 equipped with agitation for effecting a dispersion or repulping of the solids which can be transferred in the form of a liquid slurry to the autoclave 10. The liquid phase oxidation reaction can be performed at temperatures ranging from room temperature up to about 250 C. or higher with corresponding pressures permissible with the equipment being employed and in the presence of free oxygen to effect a conversion to molybdenum trioxide which, in the presence of ammonia solution, forms a corresponding aqueous soluble ammonium molybdate.

to about 5:1, depending on the specific molybdenum content of the residualfilter cake. Theformation of the ammonium hydroxide solution can conveniently be achieved by utilizing recycled wash water from the filter 12 to which ammonia, derived from the condenser 20 of the product recovery section, is added along with suitable make-up ammonia as may be required. The liquid phase oxidation reaction is preferably performed by heating the slurry to a temperature of 150 C. while maintained under proper agitation in the presence of compression air at a pressure of 500 psig for a period of about 1 hour. The compressed air is fed to the autoclave at a rate so as to maintain the quantity of free oxygen required to con- I vert all of the molybdenum dioxide to the trioxide form in an 2.MoO2+ 02 mo.

Pressure In the case where the filter cake is subjected to a supplemental roasting operation in the presence of excess air, the

' resultant roast can be treated with additional aqueous ammonium hydroxide solution to effect an extraction of the molybdenum trioxide formed. When a liquid phase oxidation of the residual solids is performed in accordance with the process schematically shown in the preparation section of FIG. 1 of the drawings, the resultant slurry either on a batchwise or continuous basis at the completion of the oxida tion reaction is transferred from the autoclave 10 to a filter 12. The filtrate derived from the filter 12, which typically contains from about 100 to about 150 grams per liter of molybdenum, is conveniently recycled back to the dissolver 2 while the solids are transferred to the scavenging section of the process. It is preferred that the filter cake in the filter 12 by washed either with a hot water solution, that is, above about 80 C., or

alternatively, a heated weak solution of ammonium hydroxide in order to remove all residual traces of soluble ammonium molybdate.

Analyses conducted on typical roasted concentrates processed in accordance with the arrangement illustrated in the preparation section of the drawing reveals that generally about 99 percent of the molybdenum present in the original concentrate is extracted in a solution form during this phase of the process. On the other hand, the filter cake derived from the filter 12 contains substantially all of the lead contaminant as well as other contaminating elements such as iron, calcium, and bismuth present in the original roasted concentrate. Analyses of typical filter cakes, based on several runs reveals the filter cake from the filter 12 to contain from 94 to 100 percent of all of the lead originally present in the roasted concentrate, while retentions ranging from about 53 to 90 percent iron, 60 to about 92 percent calcium and 89 to about 100 percent bismuth were observed during these same tests. These filter cakes further contained silica and alumina.

In accordance with an alternative satisfactory embodiment of the process comprising the present invention, the extraction and recovery of the predominant portion of the molybdenum values in the filter cake derived from the filter 6 (FIG. 1) can be achieved by subjecting the wet filter cake to a physical separation process in accordance with the schematic arrangement shown in FIG. 2 of the drawings. As shown, a repulping of the filter cake derived from the prior filtration step is conveniently achieved in a tank 8' an aqueous slurry having a solids concentration selected so as to provide for optimum physical extraction of the particles rich in molybdenum dioxide by various screening and particle classification techniques.

, Generally, slurries having a liquid-to-solids weight ratio of up to about 2:1 can be satisfactorily employed, while a liquid-tosolids ratio of about 3: l is usually preferred for most screening and classification techniques. The molybdenum values in the filter cake are predominantly in the form of molybdenum dioxide which, for most roasted concentrates, are found concentrated in the larger size particles of a size usually 150 mesh and greater.

In accordance with the process as shown in FIG. 2, the aqueous slurry is transferred from the tank 8 to the physical separation unit 10 in which the slurry is subjected to a wet screening process in which particles retained on a 150 mesh screen are recovered and typically have been found to provide a high quality molybdenum dioxide product containing around percent by weight molybdenum dioxide. Alternative classification techniques of the types well known in the art can be satisfactorily employed for extracting the particles of a size greater than about mesh including, for example, cyclonic type separators as well as separators relying on differences in the densities of the various compounds to recover those rich in molybdenum values which may be employed in lieu of or in combination with conventional screening techniques.

The large dense solids extracted in the physical separation unit 10', as shown in FIG. 2, are transferred to a filter 12' in which the water is removed. A portion of the filtrate is recycled back to the tank 8 for repulping new filter cake, while the remainder is combined with the fine-sized solids of less than about 150 mesh from the physical separation unit 10' and are transferred to a tank 24 in the scavenging section for the purposes and in accordance with the processing conditions as subsequently herein described.

The solid molybdenum dioxide product derived from the filter 12' which conventionally contains about 85 percent by weight molybdenum dioxide in combination with the molybdenum trioxide converted to soluble ammonium molybdate in the dissolver unit 2 conventionally comprise from about 97 to 98 percent of the total molybdenum value present in the roasted concentrate feed material. The extracted molybdenum dioxide product derived from the filter 12 is of suffi cient purity to enable its use such as in the manufacture of ferro-molybdenum, providing thereby a process by-product or, alternatively, the extracted molybdenum dioxide can be recycled back to a roaster and heated in the presence of excess air to effect a conversion thereof to the trioxide form. The re-roasted molybdenum oxide material can be reprocessed by combining it with new feed material charged to the dissolver unit 2 for extraction as an ammonium molybdate compound in accordance with the process previously described in connection with FIG. 1. The fines transferred from the preparation section to the scavenge section of the process which contain the predominant portion of the other contaminating metals are further processed in a manner subsequently. to be described such that the waste residuary solids discarded contain less than 1 percent molybdenum, representing an overall molybdenum conversion and recovery usually exceeding 99.8 percent based on the original material charged.

Referring now to the product recovery section in FIG. 1 of the drawings, the combined filtrates from the settling tank 4 and filter 6 are processed through a filter 14, which is efiective to remove residual solids remaining in the filtrates and/or which have precipitated from solution during transfer. The solids recovered from the filter 14 are recirculated to the dissolver 2 for further contact with the ammonium hydroxide dissolving solution. The filtrate from the filter 14, which is clear and substantially devoid of any suspended solids, typically contains from about 250 to about 270 grams per liter of molybdenum in the form of the molybdate ion. This filtrate is transferred to a precipitator or crystallizer 16 in which a concentration of the solution occurs and a precipitation and recovery of the molybdenum in the form of ammonium paramolybdate [(NHQ M 0 4H O] which is formed at crystallizer temperatures of about 50 C. or below or ammonium dimolybdate [(Nl-l Mo O,] when the crystallizer is maintained at a temperatures of 75 C. or above. Conventionally, it is preferred to control the crystallizer so as to produce predominantly ammonium paramolybdate crystals in view of their greater size and resulting ease in effecting separation and subsequent handling thereof.

The vapors from the crystallizer are transferred to a condenser 20 which is vented and the ammonia recovered therefrom as an aqueous solution is conveniently recycled to the dissolver and/or to the tank 8 of the preparation section.

The liquid in the form of a slurry saturated with ammonium molybdate crystals, is transferred from the base of the crystallizer in a continuous or batchwise manner to a centrifuge for effecting an extraction of the crystals, which thereafter are transferred to the inlet side of a calciner 22. The liquid removed in the separator 18 is returned to the input to the crystallizer 16. The calciner 22, which preferably is in the form of a rotary kiln, is heated to an elevated temperature sufficient to effect a decomposition of the ammonium molybdate crystals and a liberation of ammonia and water in the form of vapors leaving the residual molybdenum trioxide product. Conventionally, temperatures in the range of from about 400 to about 500 C. have been found satisfactory for this purpose, requiring treatment periods of approximately 1 hour to achieve the conversion of the ammonium molybdate to the corresponding molybdenum trioxide product. The vapors from the rotary kiln calciner 22 may be transferred to the condenser 20 for effecting a recovery thereof while the molybdenum trioxide product is recovered from the output end of the rotary kiln and conventionally is of a purity in excess of 99 percent and more usually of about 99.5 percent molybdenum trioxide. The foregoing corresponds to a molybdenum content of about 66.3 percent by weight. The major impurities remaining in the improved purified molybdenum trioxide product are zinc, iron and potassium, while lead contents conventionally are less than about 20 ppm. The purified molybdenum trioxide product recovered represents approximately 99 percent of the molybdenum content present in the original roasted concentrate feed material.

In order to further improve the economics of the purification process, the washed filter cake derived from the filter 12 is processed through the scavenging section, as shown in FIG. 1 of the drawings, to recover at least a portion of the residuary molybdenum content therein. Generally, such filter cakes derived from the preparation section contain about percent molybdenum, which comprises about 1 percent of the molybdenum values originally present in the roasted concentrate feed material. It has been found that by recovering upwards of 50 percent of the molybdenum in the filter cake, substantial further improvements are attained in the economics of the refining process. The fines derived from the physical separation unit generally contain about 2 to 3 percent of the original molybdenum values which are predominantly converted to a calcium molybdate by-product in accordance with the processing cycle hereinafter described, effecting thereby a recovery of upwards of about 99.6 percent of the molybdenum values present in the original feed material. The molybdenum values in the filter cake are present primarily as molybdates of the original contaminating metals, such as calcium molybdate and lead molybdate, while additional molybdenum is present as molybdenum dioxide and as undissolved or dissolved molybdenum trioxide entrapped within the filter cake.

Extraction of the major portion of the residual molybdenum values in the filter cake and/or in the fines is achieved, as shown in FIG. 1 of the drawings, by charging the solids to a tank 24 equipped with agitation and heat for forming an aqueous slurry containing from about 20 to 40 percent solids. The slurry preferably is of a liquid-to-solid ratio of about 2:1. While the temperature of the aqueous slurry is not critical and may range from 25 to l00 C., in accordance with the preferred practice, the slurry is heated during agitation to an elevated temperature ranging from about to C. and sodium carbonate in the form of a concentrated solution is added to the slurry so as to provide a theoretical 2:l molar ratio of sodium carbonate-to-molybdenum. The 100 percent excess sodium carbonate present serves to shift the equilibrium reactions toward completion in accordance with the following typical equations:

Heat

Na-ZMOO. PbCOB l The foregoing reactions are further enhanced when the temperature of the slurry is maintained at an elevated temperature, such as about 95 C., which can be satisfactorily achieved by the addition of live steam to the slurry. The addition of such live steam further serves to strip any residual ammonia in the slurry. It will be understood that alternate carbonate compounds can be employed which form soluble molybdates of which the alkali metal carbonates, and particularly potassium carbonate, can be used in lieu of or in combination with sodium carbonate. Agitation of the heated slurry is continued for a period of time sufficient to effect a stripping of all residual ammonia from the slurry and to effect a conver sion of the predominant portion of the molybdenum in the filter cake to sodium molybdate. Various tests conducted on typical filter cakes derived from roasted concentrates revealed a leaching or conversion of the molybdenum oxide content to soluble sodium molybdate in amounts generally ranging from about 52 to about 57 percent of the molybdenum present by agitating the slurry at a temperature between 95 C. and l00 C. under atmospheric pressure for periods ranging from 1 up to 4 hours.

The resultant slurry is thereafter fed from the tank 24 to a reactor autoclave 26, in which it is heated to an elevated temperature under pressure in the presence of free oxygen, to effect an oxidation of any residual molybdenum dioxide to molybdenum trioxide and to further solubilize the calcium and lead molybdates to sodium molybdate. Also, residual traces of ammonia are removed through a continuous bleeding of the vapors and gaseous products in the autoclave. Satisfactory oxidation and solubilization of such residual molybdenum values have been attained at temperatures of about C. and at compressed air pressures of 200 psig for periods of about onehalf hour. The slurry is continuously agitated in the reactor during the course of the oxidation solubilization reaction such that the molybdenum trioxide formed is converted to sodium molybdate, further increasing the quantity of leached molybdenum in solution. Typical autoclave oxidation reactions conducted on partially leached filter cakes derived from the process tank 24 were effective to further increase the percentage of molybdenum leached from the filter cake to within a range of 64 to 82 percent. In each of such runs, an excess of sodium carbonate was employed with molar ratios ranging from l.5:l up to about 8:1. These tests indicate that particularly satisfactory results are attained when the sodium carbonate-to-molybdenum ratio is controlled at about 2:1, providing for acceptable high yields employing moderate temperatures in the autoclave reactor.

The hot aqueous reaction slurry is transferred from the autoclave 26 to a filter 28, in which the residual insoluble contaminating substances are removed and are discarded to waste. Conventionally, the filter cake contains a minimal quantity of residual molybdenum which can be still further reduced by subjecting the filter cake to further processing. Usually the filter cake contains less than about 1 percent molybdenum which comprises less than about 0.2 percent of the molybdenum value present in the original roasted.concentrate feed and can be discarded without detracting from the economics of the refining process. The filter cake further contains the predominant portion of the lead, iron, calcium and bismuth contaminating "materials. Typical fractions of such contaminating elements present in the final filter cake based on the total amount present in the roasted concentrate feed, as determined by five separate runs, are 93 to 98 percent lead, 53 to 90 percent iron, 60 to 88 percent calcium and 89 to 100 percent bismuth.

The filter cake is thoroughly washed with'hot waterin the filter 28 to remove last traces of soluble sodium molybdate entrapped therein and the resultant hot wash solution is combined with the filtrate and transferred to a neutralizer 30. Usually, under'the conditions as previously defined,,the combined stream contains from about 13 up to about 37 grams per liter of dissolved molybdenum in the form of sodium molybdate, is of a pH ranging from about 9 to 10 and has a molar, ratio of sodium carbonate-to-molybdenum of about lzl. it is necessary to remove the carbonate ion from the solution prior to the removal of the molybdenum to avoid contamination of the calcium molybdate with calcium carbonate. This is achieved in the neutralizer by adding a hydrochloric acid solution to the heated filtrate, effecting a reaction in accordance with the following equation:

Na CO 2HCl 2NaCl H,O+ CO l The neutralization of the filtrate is continued and live steam is preferably added to maintain the filtrate in a heated condition, while simultaneously facilitating a stripping of the carbon dioxide formed during the reaction. 'The addition of hydrochloric acid is continued until the evolution of carbon dioxide ceases and a pH ranging from about 6.75 to about 7.25 is attained. Neutralization can also be achieved by using other acids such as nitric acid for example.

The neutralized filtrate is thereafter transferred to a precipitator 32 equipped with suitable agitation and a calcium chloride solution is added in an amount so as to obtain a 1:1 molar ratio of molybdenum-to-calcium, thereby forming the equivalent calcium molybdate. The reaction proceeds rapidly and the calcium molybdate formed precipitates from the solution in the form of fine-sized white particles usually averaging about 2 microns in size. During the precipitation step, the solution is preferably maintained at a temperature about 80 C. Besides calcium chloride which comprises a preferred material in view of economics and the utility of calcium molybdate as a steel addition agent or as an inhibitive pigment in coating compositions, other alkaline earth compounds can be used provided that they form an insoluble molybdate compound. Typical of such alternative materials are ZnCl BaCl and SrCl The precipitation step is continued until substantially all of the molybdenum has been extracted as insoluble calcium molybdate, which generally requires a period of about onehalf hour to achieve. During the precipitating operation, the pH of the solution rises and a recovery of about 99.9 percent of the molybdenum therein is attained when the pH is above about 6.5. The resultant precipitate is extracted from the residual solution by a filter 34 and the filtrate, which contains sodium chloride and a minimal quantity of molybdenum, such as 0.010 to about 0.035 grams per liter, is discarded to waste. The resultant filter cake comprising a calcium molybdate product is transfered from the filter 34 to a dryer 36 in which substantially all of the moisture is removed, producing a calcium molybdate by-product. This by-product molybdenum.

comprises up to about 1 percent of the molybdenum'content present in the original roasted concentrate and may be conveniently processed in accordance with other well known techniques or used as a feed, such as, for example, for ferromolybdenum production. It is also contemplated that the calcium molybdate by-product obtained from the dryer 36 or in a wet state directly from the filter '34 can be transferred to the calciner 22' for admixture with the'ammonium molybdate product recovered from the separator 18. in accordance with this alternative practice, a technical grade molybdenum oxide product is formed of exceedingly low lead content and is suitable for use in those instances in which the presence of varying proportions of calcium are not objectionable.

It will be apparent from the foregoing that the process comprising the present invention provides an economical means for producing a molybdenum trioxide product of high purity containing only minimal amounts of such contaminating elements as lead, bismuth, silicon, calcium and aluminum and providing for a recovery of molybdenum based on the original quantity present in the feed of upwards of 99percent and generally upwards of 99.5 percent.

While it 'will be apparent that the description of the preferred embodiments of the process comprising the present invention will achieve the benefits as herein set forth, the invention is susceptible to modification, variation and change without departing from'the spirit thereof.

What is claimed is:

1. A process'for producing a refined molybdenum oxide product which comprises the steps of contacting a crude molybdenum oxide material of a size predominantly less than about 50 mesh with a first aqueous ammonium hydroxide solution having a concentration so as to provide a molar ratio of ammonium hydroxide-to-molybdenum of greater than 2:l for a period of time sufficient to convert a major portion of the molybdenum trioxide content to a first soluble ammonium molybdate solution, separating said first solution from the undissolved solid residue, oxidizing said residue to convert at least a portion of the remaining molybdenum content thereof to additional molybdenum trioxide, contacting the oxidized said residue with a second aqueous ammonium hydroxide solution to convert at least a portion of said additional molybdenum trioxide to additional soluble ammonium molybdate solution, separating the ammonium molybdate from the remaining reoxidized residue, combining said first and said ad ditional ammonium molybdate solutions, separating the ammonium molybdate from. the combined solutions as solid crystals, and calcining said crystals at an elevated temperature for a period of time sufficient to liberate ammonia and water, forming a refined molybdenum trioxide product, contacting said insoluble residue remaining after said oxidation step with an alkali metal carbonate to form an aqueous soluble alkali metal molybdate, separating the solution containing the dis solved alkali metal molybdate from the insoluble reoxidized residue and thereafter extracting the molybdate from said solution as a calcium molybdate precipitate.

2. The process as defined in claim 1, wherein said oxidizing of the said residue is achieved by a heating thereof to an elevated temperature in the'presence of air under pressure while dispersed in an aqueous solution.

3. The process as defined in claim 1, wherein said oxidizing of said residue is achieved by a roasting thereof in the presence of air at an elevated temperature.

4. The process as defined in claim 1, including the further step of contacting said aqueous ammonium molybdate solutions with sulfide compounds to effect a precipitation and extraction of soluble copper and zinc contaminants as insoluble sulfides. v

5. The process as defined in claim 1, including the further steps of physically classifying the insoluble said residue remaining after separating said first ammonium molybdate solution and separating that portion of the larger sized particles containing the predominant portion of the molybdenum values in said residue.

6. The process as defined in claim 5, wherein at least a portion of said larger particles are subjected to oxidation by a roasting thereof in the presence of air at an elevated temperature.

7. The process as defined in claim 5, including the further step of contacting the residual small sized particles with a solution containing an alkali metal carbonate to form a soluble alkali metal molybdate which subsequently is recovered in the form of a precipitated alkaline earth metal molybdate. 

2. The process as defined in claim 1, wherein said oxidizing of the said residue is achieved by a heating thereof to an elevated temperature in the presence of air under pressure while dispersed in an aqueous solution.
 3. The process as defined in claim 1, wherein said oxidizing of said residue is achieved by a roasting thereof in the presence of air at an elevated temperature.
 4. The process as defined in claim 1, including the further step of contacting said aqueous ammonium molybdate solutions with sulfide compounds to effect a precipitation and extraction of soluble copper and zinc contaminants as insoluble sulfides.
 5. The process as defined in claim 1, including the further steps of physically classifying the insoluble said residue remaining after separating said first ammonium molybdate solution and separating that portion of the larger sized particles containing the predominant portion of the molybdenum values in said residue.
 6. The process as defined in claim 5, wherein at least a portion of said larger particles are subjected to oxidation by a roasting thereof in the presence of air at an elevated temperature.
 7. The process as defined in claim 5, including the further step of contacting the residual small sized particles with a solution containing an alkali metal carbonate to form a soluble alkali metal molybdate which subsequently is recovered in the form of a precipitated alkaline earth metal molybdate. 